Proportional pressure control solenoid valves are used in many mobile and industrial hydraulics applications. In these type of hydraulic solenoid valves, an electromechanical mechanism of the device produces an output force that is a function of the applied current signal. The output force controls the hydraulic mechanism of the device, which throttles hydraulic pressure supplied by a remote source and provides a controlled pressure at the output port of the device. With the widespread utilization of microcomputers on passenger cars and trucks, the ability to apply proportional pressure control solenoid valves to automotive applications has grown. This type of valve is potentially beneficial in controlling fluid flow in automatic transmissions, power steering and braking systems.
A proportional pressure control solenoid valve typically includes a valve spool that is linearly movable in a cylindrical bore formed within a housing. Output or control pressure may be applied to the end of the valve spool through an external conduit or through passages drilled in the housing. When this control pressure is applied to the valve spool, a feedback force that is proportional to the control pressure is generated which opposes the force of a modulating spring positioned on the opposite end of the spool. Feedback area is directly related to spool diameter and is limited by manufacturing restrictions on the valve spool. A magnetic force is typically created by an electro-mechanical portion of the solenoid. The magnetic force is typically applied in opposition to the force applied by the modulating spring. The magnetic force increases when the current applied to the solenoid is increased. Varying the magnetic force changes the force balance established by the modulating spring and the feedback force, which causes the valve spool to move to re-establish a force equilibrium. Spool movement or "modulation," throttles the inlet pressure and provides a controlled pressure at the output port.
With this type of valve, size, cost and mass are ever-present design factors that are preferably minimized. These factors can be reduced if the magnetic force requirements are reduced. However, the modulating spring force and feedback force are limiting, acting as a boundary restricting the ability to reduce the aforementioned factors. Since the feedback force is proportional to the control pressure and the area of the valve spool, a pressure limitation exists, creating a desire to make the spool as small as possible. The spool's minimum size is restricted by cost and manufacturing limitations. Therefore, proportional pressure control solenoid valves are typically limited to lower pressure applications, since high pressure applications would require excessive efforts to achieve desirable cost and size. Additionally, prior art proportional pressure control solenoid valves may require costly design features or components to provide viscous dampening to control undesirable operating characteristics.